1. Field of the Invention
The present invention relates generally to the field of lasers, and more specifically to controlling a pulsed laser.
2. Related Art
Chirped Pulse Amplification (CPA) is very useful for producing ultra-short duration high-intensity pulses for use in high peak power ultra-short pulse laser systems using an optical amplifier. CPA increases the energy of an ultra-short laser pulse while avoiding optical amplifier damage and excessive nonlinear distortion. In this technique, the duration of the pulse is increased by first dispersing the ultra-short laser pulse temporally as a function of wavelength (a process called “chirping”) to produce a chirped pulse, then amplifying the chirped pulse, and then recompressing the chirped pulse to significantly shorten its duration. Stretching the pulse or lengthening the pulse in time reduces the peak power of the pulse and, thus, allows energy to be added to the pulse without incurring excessive nonlinearities or reaching a damage threshold of the optical amplifier and optical components. The amount of pulse amplification that can be achieved is typically proportional to the amount of pulse stretching and compression. Typically, the greater the amount of stretching and compression, the greater the possible pulse amplification.
FIG. 1 is a block diagram illustrating a prior art chirped pulse amplification (CPA) system 100. The CPA system 100 includes a pulsed laser 102, a pulse stretcher 104, an optical amplifier 106, and an optical compressor 108. The pulsed laser 102 generates a pulsed laser signal 110. The pulse stretcher 104 temporally disperses the pulsed laser signal 110 while reducing a peak power of the pulsed laser signal 110 to generate a stretched pulsed laser signal 112. The optical amplifier 106 amplifies the stretched pulsed laser signal 112 to generate an amplified pulsed laser signal 114. Finally, the optical compressor 108 temporally compresses the amplified pulsed laser signal 114 to generate an amplified ultra-short pulsed laser signal 116 at an output of the CPA system 100. The pulse stretcher 104 and the optical compressor 108 are typically configured to have approximately equal but opposite dispersive properties to compensate for one another and minimize a pulse width of the amplified ultra-short pulsed laser signal 116.
Many end-use applications of the CPA system 100 may require an output pulse repetition rate to be controlled. For example, the output pulse repetition rate may need to be lower than a lower limit of the repetition rate of the stretched pulsed laser signal 112 input to the optical amplifier 106 which enables the optical amplifier 106 to maintain stable operation. For some end-use applications, each output pulse may need to be individually triggered. An approach to provide these low repetition rate or individually triggered output pulses is to provide a high power pulse picker (not shown), such as a modulator or an acousto-optic switch, between the optical amplifier 106 and the output of the CPA system 100. The high power pulse picker may be configured to select high power pulses as needed to provide the desired low repetition rate or individually triggered output pulses.
In addition to additional system complexity, the high power pulse picker may have other disadvantages. The high power pulse picker may have low efficiency, which may typically be less than 80%. The high power pulse picker may cause instability of the CPA system 100 and spatial distortions of an output optical beam. The high power pulse picker including its radio-frequency (RF) electronics and power supply may also be bulky and require a significant amount of space relative to the rest of the CPA system 100. Furthermore, the high power pulse picker may create additional heat due to its RF electronics and load.
Accurately directing individually triggered output pulses to a target may be difficult because an exact point of impingement of an output pulse on the target may not be known until after the output pulse impinges on the target. Because each output pulse may have sufficiently high energy which may cause effects such as ablation at the point of impingement on the target, it is desirable for the output pulse to impinge only at the desired target.